She could be a Wise Woman Hero of the Horn. Caught Avi in the Dream and got Rand when the horn was blown.

For all those that don't believe the Seanchan could defeat the combined armies of the west, think of this scenerio:

All the armies and aiel fight the trollocs and are slaughtered or at least reduced in strength considerably before Rand seals the bore. Many of the remaining armsmen go back to their farms or to settle the new lands.

Due to the infinite wisdom of the Empress, the Seanchan do not commit all of their forces and suffer much smaller losses. In the disheaval after the loss of Rand, they are the most well equiped and organized bunch.

Even so they would need a lot of lucky breaks, but it is quite possible.